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Structural Traps in Duplexes Within the Pine Mountain Thrust System, Southern Appalachians

Shankar Mitra

The Rose Hill and Ben Hur oil fields in the Pine Mountain thrust system are classic examples of structural traps formed by duplexes in the southern Appalachian thrust belt. Integration of surface, seismic, and well data has enabled the construction of eight balanced cross sections and an interpretation of the three-dimensional geometry and kinematic development of structural traps in the area. In the southwestern and northeastern segments of the Pine Mountain thrust block, the structure consists of a simple ramp-related anticline, with the shortening decreasing from 70,000 to 10,000 ft toward the northeast. In the central segment, the Pine Mountain thrust is folded by one or more footwall thrusts, resulting in a system of overlapping ramp anticlines (class III duplex), wi h the total shortening varying between 54,000 and 28,000 ft. Structural traps in the Rose Hill and Ben Hur oil fields occur within Ordovician and Silurian units in the crest of the Bales I thrust sheet. These traps formed in areas with the maximum structural relief prior to refolding by subsequently formed footwall thrusts. The porosity and permeability of the reservoirs are enhanced by fold-related longitudinal and transverse fractures at high angles to bedding. Faults are typically characterized by a few inches of gouge surrounded by a brecciated and imbricated permeable zone several feet or tens of feet wide. This suggests that while the faults acted as impermeable barriers between different thrust sheets, they also provided effective pathways for communication within each sheet durin the migration of hydrocarbons.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.